Well I was wrong. The rubber exhaust dealios are 3" according to the pictures I just took during my lunch hour (see below - The measurement was taken from an angle so it may look smaller than 3", but the rubber dealio is 3"). So it seems to me that a 3" unit would work all the way through, since ETDUC says the exhaust pipe has a 3" I.D. Of course these tips come with fewer screw holes so like ETDUC says, the existing holes will need to be sealed prior to putting the new one on.
Maybe this is more trouble than its worth.
Price: Now that we know we are using a 3" dia unit, I don't see one on the Rex site, only reducers, so I will have to email them. Hardin has the 3" units with rubber flappers. Hardin did tell me:
"I can tell you that manufactures recommendations are that if the exhaust tip is within 12" of the waterline which I believe yours is that you should run two flaps a internal and external. That being said the angle tip will not accommodate an external flap. Don't get me wrong you can buy any tip you chose I just want you to know some facts
All of our tips are build with a serviceable rubber flaps that should be replaced during annual services. For years we built tips with stainless butterfly style flaps but the "Clanking and banging" sounds caused by stainless flaps lead to extreme customer complaints and caused us to cancel that design. As for flaps falling out we do not experience that, we invested hundreds of hours in not only the compound but the flap cage design when this product was designed. The sure fire way to kill any rubber flap is extensive heat without water. If you plan to run the boat dry for extensive periods of time that's the only sure fire way to kill the flap.
The external flaps are just added protection against water surges when the engines off"